This invention relates to an image forming device using an electrophotographic process and, in particular, to a digital color image forming device using a color developer.
Recently, color image forming devices using an electrophotographic process are rapidly changed from those using an analog color image forming method based on a conventional analog copying machine into those using a digital color image forming method which carries out digital processing of color data for each color in compliance with material and process characteristics of each factor composing an image forming process and which enables a wide variety of image edition.
As regards the output on various types of recording media, which is a characteristic of the electrophotographic process, digital full-color images can be produced on thick plain paper, thin plain paper, secondary stencil paper, OHP sheets and so on.
However, it is impossible to achieve a target image quality if color images are produced on such various types of recording media under a common process condition. Most of the devices cope with this problem by selecting different conditions in each process in dependence upon the types of the recording media.
A most popular approach is to select different fixing conditions for the recording media after transfer. This includes variation of a process speed down to a transfer process and a process speed in a fixing process in dependence upon the recording media, variation of a temperature, variation of a fixing pressure, variation of a carrier angle of the recording media with respect to a fixing unit, and a combination thereof.
As another approach in a process other than the fixing process, use is made of variation of transfer pressure, and bias conditions in development, transfer and separation.
In the above-mentioned approaches which have been practiced or proposed, however, the same latent image is formed on a photosensitive member irrespective of the type of the recording medium. In most of those approaches, variation is made in the development, the transfer, or the fixing processes thereafter, particularly in the fixing process.
In the image forming technique, particularly in the electrophotographic process, image formation is generally carried out on a balance throughout the whole process. Such balance, in other words, optimization in each process is decided on the basis of standard plain paper of 64 g/m.sub.2. For other recording media, optimization is carried out by uniform modification of the decided condition in each image forming process, for example, variation in fixing speed.
Specifically, for output on the OHP sheets and the thick plain paper, the fixing process is carried out at a fixing speed corresponding to a half of an ordinary process speed. However, the images formed on the OHP sheets and the thick plain paper are observed in different manners, namely, as transparent images and as reflected images, respectively. With respect to the image quality, it is insufficient to form those images under the same image forming condition. Although the fixing speeds are different between the plain paper and the OHP sheets, no difference is present in developed images formed on the photosensitive member. Because of differences in surface condition, glossiness, and chromaticity of the recording medium itself, a final image has a different image quality dependent upon the type of the recording medium. On the other hand, the above-mentioned approach using variation of the bias in each process is closely related to an environment. In addition, the variation is uniform irrespective of the content of the image. Therefore, there remains a difficulty in achieving a uniform and stable image quality for various recording media.